1. Field of the Invention
The present invention relates to a magneto-optical recording medium including an exchange coupling magnetic layer essentially consisting of a recording layer and a reading layer, which attains high density recording by transferring the direction of magnetization in the recording layer to the reading layer for reading, and to manufacturing method thereof.
2. Description of the Background Art
A magneto-optical recording medium has attracting attention as a recording medium which is rewritable and having large storage capacity and high reliability. It has come to be used as a computer memory, for example. However, as the amount of information increases and the device is made smaller, recording/reading technique with higher density has been desired.
The technique for recording and reading with high density includes techniques related to the apparatus for recording and reading, and techniques for the medium. The former techniques include an optical super resolution technique for obtaining a convergence spot exceeding diffraction limit of laser beam, and a technique for making shorter the wavelength of the laser beam. The latter techniques include narrowing the pitch of the medium, and improvement of the reading resolution by using magnetic multi-layered films. The technique for improving the reading resolution by using a magnetic multi-layered film includes a technique in which the state of the recording layer is selectively transferred to the reading layer and the state of the reading layer is read, utilizing the fact that temperature distribution of the laser spot has Gaussian distribution. In a magneto-optical recording medium used for the optical super resolution technique, a recording layer which is magnetized in a direction perpendicular to the disc surface has been generally used. As a substrate for the medium, generally, a glass substrate has been used.
In NIKKEI ELECTRONICS 1993, 7.5 (No. 585) pp. 163-170, a magneto-optical disc including a magnetic film consisting of a reading layer and a recording layer is disclosed. The conventional magneto-optical disc disclosed in this paper will be described. FIG. 1 shows the structure of the disclosed medium. On a glass substrate 101, an APN dielectric layer 102 having the thickness of 80 nm, a GdFeCo reading layer 103 having the thickness of 40 nm, a DyFeCo recording layer 104 having the thickness of 40 nm, an APN dielectric layer 105 having the thickness of 20 nm, and a protective layer 107 are formed in this order. In the magneto-optical disc, the magnetic material of which the recording layer is formed is magnetized in the perpendicular direction and records information. Meanwhile, the reading layer is formed of a magnetic film in which in-plane magnetization occurs at room temperature and perpendicular magnetization occurs at high temperature. The principle of reading from a magneto-optical disc using such an in-plane magnetic film as the reading layer is as shown in FIG. 2. When the magneto-optical disc is irradiated with laser beam 17, intensity of the laser spots has Gaussian distribution. Therefore, temperature distribution of the magnetic film also tends to have Gaussian distribution, as shown in the figure. Since the magnetic material of which the direction of magnetization changes from in-plane to perpendicular direction when the temperature exceeds 120.degree. C. is used as the reading layer, only the area 13a of reading layer 13 where the temperature exceeds 120.degree. C. because of laser beam 17 is magnetized in the perpendicular direction, in accordance with the direction of magnetization of the corresponding portion of recording layer 14. Thus, the information of recording layer 14 is transferred to reading layer 13, enabling reading of the information in recording layer 14. Reading of information from the transferred portion is realized, utilizing Kerr effect. Kerr effect refers to the effect that when light is reflected from a magnetic body, the plane of polarization of the reflected light is rotated. In the magneto-optical disc, the rotation angle is read as an output signal. As shown in FIG. 3A, when reading layer 103 which is in the state of perpendicular magnetization is irradiated with light, the Kerr rotation angle on the plane of polarization becomes maximum. Meanwhile, as shown in FIG. 3B, when reading layer 103 in the state of in-plane magnetization is irradiated with light, there is hardly a rotation of the plane of polarization. Referring to FIG. 2, at the center of the spot of laser beam 17 where temperature is high, magnetization of reading layer is perpendicular, and at other portions, magnetization is in-plane. Therefore, at the center of the spot, the aforementioned Kerr effect occurs, while at other portions, Kerr effect does not occur. Therefore, only the signal at the center of the spot can be detected, and other portions are masked by in-plane magnetization. As shown in FIG. 2, the area where the threshold value 120.degree. C. is exceeded can be made smaller than the laser spot diameter. Accordingly, information of the area which is smaller than the laser spot diameter can be read, which means that a higher recording density is possible. Such an effect is referred to as magnetically induced super resolution (MSR). As to writing of information to the recording layer, when a portion of the recording layer which has been heated to a temperature not lower than Curie temperature by the laser beam comes to be off the laser beam and the temperature thereof becomes lower than Curie temperature, it is magnetized in the direction of an external magnetic field. Recording of information by perpendicular magnetization is performed in this manner. In magnetic field modulation recording with continuous irradiation, the medium is continuously irradiated with the laser beam at the time of recording. Meanwhile, in laser pulse magnetic field modulation recording, pulse laser is emitted, and the cycle of heating to at least Curie temperature and cooling is repeated in an instant during recording.
SPIE Vol. 1316 Optical Data Storage (1990)/271-277 discloses a method of high density magneto-optical recording using magnetic field modulation and pulsed laser irradiation. In the magneto-optical disc disclosed in this paper, a TbFeCo recording layer having Curie temperature of 180.degree. C. is formed on a glass substrate.
The method of detection for MSR includes a center aperture detection (CAD), a front aperture detection (FAD) and a rear aperture detection (RAD). In the CAD, information of the recording layer is read through the reading layer at the central portion of the laser beam spot irradiated for heating. In the FAD, as shown in FIG. 4A, a portion 110a in front of the disc proceeding direction is masked at the laser beam spot 110, and other portions are opened. In the RAD, as shown in FIG. 4B, a portion 110b in front of the disc proceeding direction of the laser beam spot 110 is opened and other portions are masked. JJAP Series 6 Proc. Int. Symp. on Optical Memory, 1991, pp. 203-210 discloses a structure of a magnetic film for the MSR disc utilizing the FAD and RAD. In the MSR disc using the FAD disclosed in this paper, a GdFeCo read out layer having the thickness of 30 nm and having the Curie temperature of at least 300.degree. C., a TbFeCoAl switching layer having the thickness of 10 nm and the Curie temperature of about 140.degree. C., and a TbFeCo recording layer having the thickness of 40 nm and the Curie temperature of about 250.degree. C. are stacked. In the MSR disc using the RAD, a GdFeCo read out layer having the thickness of 30 nm and the Curie temperature of at least 300.degree. C., a TbFeCoAl subsidiary layer having the thickness of 10 nm and the Curie temperature of about 140.degree. C., a GdFeCo intermediate layer having the thickness of 15 nm and the Curie temperature of about 250.degree. C., and a TbFeCo recording layer having the thickness of 40 nm and the Curie temperature of about 250.degree. C. are provided.
In the magneto-optical disc in which the direction of magnetization of the recording layer is transferred to the reading layer at the time of reading, the transfer characteristic is of critical importance. It is desired that the transfer is performed quickly at a prescribed temperature attained by heating. However, the conventional magneto-optical disc still has a room for improvement in the transfer characteristic. In the conventional magneto-optical disc of the CAD method, there is a possibility of conversion from in-plane magnetization perpendicular magnetization in the reading layer in a relatively wide temperature range of several tens to about 100.degree. C. Such a transfer in a wide temperature range causes much reading noises results in an insufficient MSR effect. The wider the area of the reading layer magnetically influenced by the recording layer at the time of reading, the lower the mask effect, and it becomes more difficult to clearly read the information recorded with high density. In addition, the transfer temperature of the magnetic film much depends on various conditions for forming the film. Therefore, it is relatively difficult to form a magnetic film having a definite threshold value for the transfer with good reproductivity.
In order to heat the recording layer and the reading layer at the time of recording to a temperature not lower than the Curie temperature, conventionally, it was necessary to increase laser power. When heating is not sufficient, a carrier to noise ratio (CNR) of the recording signal is degraded. Further, in order to align the direction of magnetization of the not sufficiently heated reading layer to the direction of the external magnetic field, it is necessary to apply a large magnetic field. In this case also, the CNR of the recording signal is degraded. Further, in magnetic field modulation recording, it is desired that a smaller magnetic field be applied.
When the Curie temperatures of the recording layer is low and the difference between the Curie temperatures of the recording layer and the reading layer is large, sometimes a part of the magnetization of the reading layer prematurely turns to the in-plane direction when the transfer of the direction of magnetization of the reading layer to the recording layer starts during the process of cooling at the time of recording. Such a phenomenon causes noises in the signal to be transferred from the reading layer to the recording layer, degrading the CNR of the recording signal.
In the technique where the state of the recording layer is transferred to the reading layer for reading, it is desired that the temperature distribution of the reading layer heated by the laser spot should have a desired distribution. The reason for this is as follows. When the temperature distribution deviates from the desired distribution, noises caused by random magnetization directions increase, and further, crosstalk noises increase which derive from reading of the state of a portion other than the center of laser spot (that is, a peripheral portion which should have lower temperature). In a transparent type magneto-optical recording medium in which light is transmitted through the magnetic layer, heat accumulation caused by the laser spot irradiation is negligible. Meanwhile, in a magneto-optical recording medium in which the laser beam reflected from a magnetic layer having the thickness of at least 400 .ANG. is detected, the heat accumulation affects the temperature distribution of the reading layer. Therefore, in the reflection type magneto-optical recording medium, the aforementioned noises are increased. Further, in the conventional magneto-optical recording medium, when recording is performed with the medium being irradiated with the laser beam of a constant intensity, the heated area in the recording layer has been larger than the laser spot. As a result, the recording spot becomes larger, making it difficult to increase the density in recording.
When glass is used as the substrate, the following problems are experienced.
a) The magneto-optical recording medium becomes heavy. PA1 b) There is a possibility of breakage when one erroneously drops the medium. PA1 c) The medium cannot withstand a high speed rotation. PA1 d) Surface polishing of the substrate requires much cost. PA1 e) It is difficult to form a guiding groove used for tracking the laser beam directly thereon.
An object of the present invention is to provide a magneto-optical recording medium of the type in which the direction of magnetization of the recording layer is transferred to the reading layer at the time of reading, which has a superior transfer characteristic, and low noises in reading and which attains a high MSR.
Another object of the present invention is to provide a technique for obtaining a magneto-optical recording medium which has a superior transfer characteristic and low reading noises and which can attain a high MSR, with high reproductivity.
A further object of the present invention is to provide a magneto-optical recording medium which allows recording even with a small magnetic field applied and which can be preferably applied to the magnetic field modulation technique.
A still further object of the present invention is to provide a magneto-optical recording medium which is capable of recording with a good CNR and reading with a good CNR.
A still further object of the present invention is to provide a magneto-optical recording medium in which a heat accumulation can be suppressed to a negligible level, and hence noises are reduced.
A still further object of the present invention is to provide a magneto-optical recording medium in which the uniformly heated area in recording and reading can be made smaller, thereby enabling high density recording and reading.
A still further object of the present invention is to provide a magneto-optical recording medium which is easy to handle, on which a guiding groove used for tracking the laser beam can be directly formed and which is relatively inexpensive.